To provide a field apparatus capable of obtaining high reliability without needing the duplex instrumentation of the field apparatus.

A main unit 1A and a sub-unit 1B exchange output value or device information (such as status information on a device), parameter written contents and data information on an EEPROM 14b within each unit mutually through a CPU 15. Also, the main unit 1A and the sub-unit 1B verify self data with data on the other unit with respect to exchanged data at the CPU 15, and determine if both are different and, by keeping comparing them for a long period, determine whether abnormal conditions are present to numerical values (such as, the occurrence of deviation over prescribed one or an extended tendency of deviation) and whether there is deviation in operation timing.